docs: add accessibility overview

docs/accessibility.md

+# Accessibility Overview
+
+This document describes how accessibility is implemented throughout Chromium at
+a high level.
+
+## Concepts
+
+The three central concepts of accessibility are:
+
+1. The *tree*, which models the entire interface as a tree of objects, exposed
+   to screenreaders or other accessibility software;
+2. *Events*, which let accessibility software know that a part of the tree has
+   changed somehow;
+3. *Actions*, which come from accessibility software and ask the interface to
+   change.
+
+Here's an example of an accessibility tree looks like. The following HTML:
+
+```
+<select title="Select A">
+  <option value="1">Option 1</option>
+  <option value="2" selected>Option 2</option>
+  <option value="3">Option 3</option>
+</select>
+```
+
+has a generated accessibility tree like this:
+
+```
+0: AXMenuList title="Select A"
+1:   AXMenuListOption title="Option 1"
+2:   AXMenuListOption title="Option 2" selected
+3:   AXMenuListOption title="Option 3"
+```
+
+Given that accessibility tree, an example of the events generated when selecting
+"Option 1" might be:
+
+```
+AXMenuListItemUnselected 2
+AXMenuListItemSelected 1
+AXMenuListValueChanged 0
+```
+
+An example of a command used to change the selection from "Option 1" to "Option
+3" might be:
+
+```
+AccessibilityMsg_DoDefaultAction 3
+```
+
+All three concepts are handled at several layers in Chromium.
+
+## Blink
+
+Blink constructs an accessibility tree (a hierarchy of [WebAXObject]s) from the
+page it is rendering. WebAXObject is the public API wrapper around [AXObject],
+which is the core class of Blink's accessibility tree. AXObject is an abstract
+class; the most commonly used concrete subclass of it is [AXNodeObject], which
+wraps a [Node]. In turn, most AXNodeObjects are actually [AXLayoutObject]s,
+which wrap both a [Node] and a [LayoutObject]. Access to the LayoutObject is
+important because some elements are only in the AXObject tree depending on their
+visibility, geometry, linewrapping, and so on. There are some subclasses of
+AXLayoutObject that implement special-case logic for specific types of Node.
+There are also other subclasses of AXObject, which are mostly used for testing.
+
+Note that not all AXLayoutObjects correspond to actual Nodes; some are synthetic
+layout objects which group related inline elements or similar.
+
+The central class responsible for dealing with accessibility events in Blink is
+[AXObjectCacheImpl], which is responsible for caching the corresponding
+AXObjects for Nodes or LayoutObjects. This class has many methods named
+`handleFoo`, which are called throughout Blink to notify the AXObjectCacheImpl
+that it may need to update its tree. Since this class is already aware of all
+accessibility events in Blink, it is also responsible for relaying accessibility
+events from Blink to the embedding content layer.
+
+## The content layer
+
+The content layer lives on both sides of the renderer/browser split. The content
+layer translates WebAXObjects into [AXContentNodeData], which is a subclass of
+[ui::AXNodeData]. The ui::AXNodeData class and related classes are Chromium's
+cross-platform accessibility tree. The translation is implemented in
+[BlinkAXTreeSource]. This translation happens on the renderer side, so the
+ui::AXNodeData tree now needs to be sent to the browser, which is done by
+sending [AccessibilityHostMsg_EventParams] with the payload being serialized
+delta-updates to the tree, so that changes that happen on the renderer side can
+be reflected on the browser side.
+
+On the browser side, these IPCs are received by [RenderFrameHostImpl], and then
+usually forwarded to [BrowserAccessibilityManager] which is responsible for:
+
+1. Merging AXNodeData trees into one tree of [BrowserAccessibility] objects,
+   by linking to other BrowserAccessibilityManagers. This is important because
+   each page has its own accessibility tree, but each Chromium *window* must
+   have only one accessibility tree, so trees from multiple pages need to be
+   combined (possibly also with trees from Views UI).
+2. Dispatching outgoing accessibility events to the platform's accessibility
+   APIs. This is done in the platform-specific subclasses of
+   BrowserAccessibilityManager, in a method named `NotifyAccessibilityEvent`.
+3. Dispatching incoming accessibility actions to the appropriate recipient, via
+   [BrowserAccessibilityDelegate]. For messages destined for a renderer,
+   [RenderFrameHostImpl], which is a BrowserAccessibilityDelegate, is
+   responsible for sending appropriate `AccessibilityMsg_Foo` IPCs to the
+   renderer, where they will be received by [RenderAccessibilityImpl].
+
+On Chrome OS, RenderFrameHostImpl does not route events to
+BrowserAccessibilityManager at all, since there is no platform screenreader
+outside Chrome to integrate with.
+
+## Views
+
+Views generates a [NativeViewAccessibility] for each View, which is used as the
+delegate for an [AXPlatformNode] representing that View. This part is relatively
+straightforward, but then the generated tree must be combined with the web
+accessibility tree, which is handled by BrowserAccessibilityManager.
+
+## WebUI
+
+Since WebUI surfaces have renderer processes as normal, WebUI accessibility goes
+through the blink-to-content-to-platform pipeline described above. Accessibility
+for WebUI is largely implemented in JavaScript in [webui-js]; these classes take
+care of adding ARIA attributes and so on to DOM nodes as needed.
+
+## The Chrome OS layer
+
+The accessibility tree is also exposed via the [chrome.automation API], which
+gives extension JavaScript access to the accessibility tree, events, and
+actions. This API is implemented in C++ by [AutomationInternalCustomBindings],
+which is renderer-side code, and in JavaScript by the [automation API]. The API
+is defined by [automation.idl], which must be kept synchronized with
+[ax_enums.idl].
+
+[AccessibilityHostMsg_EventParams]: https://cs.chromium.org/chromium/src/content/common/accessibility_messages.h?sq=package:chromium&l=75
+[AutomationInternalCustomBindings]: https://cs.chromium.org/chromium/src/chrome/renderer/extensions/automation_internal_custom_bindings.h
+[AXContentNodeData]: https://cs.chromium.org/chromium/src/content/common/ax_content_node_data.h
+[AXLayoutObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.h
+[AXNodeObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXNodeObject.h
+[AXObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXObject.h
+[AXObjectCacheImpl]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
+[AXPlatformNode]: https://cs.chromium.org/chromium/src/ui/accessibility/platform/ax_platform_node.h
+[AXTreeSerializer]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_tree_serializer.h
+[BlinkAXTreeSource]: https://cs.chromium.org/chromium/src/content/renderer/accessibility/blink_ax_tree_source.h
+[BrowserAccessibility]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility.h
+[BrowserAccessibilityDelegate]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility_manager.h?sq=package:chromium&l=64
+[BrowserAccessibilityManager]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility_manager.h
+[LayoutObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutObject.h
+[NativeViewAccessibility]: https://cs.chromium.org/chromium/src/ui/views/accessibility/native_view_accessibility.h
+[Node]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/dom/Node.h
+[RenderAccessibilityImpl]: https://cs.chromium.org/chromium/src/content/renderer/accessibility/render_accessibility_impl.h
+[RenderFrameHostImpl]: https://cs.chromium.org/chromium/src/content/browser/frame_host/render_frame_host_impl.h
+[ui::AXNodeData]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_node_data.h
+[WebAXObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/public/web/WebAXObject.h
+[automation API]: https://cs.chromium.org/chromium/src/chrome/renderer/resources/extensions/automation
+[automation.idl]: https://cs.chromium.org/chromium/src/chrome/common/extensions/api/automation.idl
+[ax_enums.idl]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_enums.idl
+[chrome.automation API]: https://developer.chrome.com/extensions/automation
+[webui-js]: https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/